1. Field of the Invention
The present invention relates to pregel compositions for use in forming electrochemically stable polymer gel electrolytes, and to a method of dehydrating such pregel compositions. The invention further relates to secondary cells and electrical double-layer capacitors containing polymer gel electrolytes prepared using such pregel compositions.
2. Prior Art
Polymer gel electrolytes prepared by using a pregel composition to gel an organic electrolyte solution of an electrolyte salt in a nonaqueous solvent are currently seeing use as electrolytes in nonaqueous secondary cells and electrical double-layer capacitors. Such polymer gel electrolytes must be electrochemically stable. This requires that both the organic electrolyte solution and the pregel composition be sufficiently free of water. Pregel compositions which have not been sufficiently dehydrated are undesirable. Also undesirable are pregel compositions which contain residual amounts of organic solvents that lower the electrochemical properties, pregel compositions which contain residual amounts of impurities that adsorb to the electrolyte, electrode surfaces or separator, and pregel compositions which have begun to polymerize and thus have a higher viscosity that keeps them from fully penetrating the separator and electrodes.
A number of possible approaches for dehydrating pregel compositions immediately come to mind, such as lowering the viscosity with a co-solvent and using a dehydrating adsorbent to effect dehydration, or removing water by means of distillation. Specific examples from the prior-art include:    (1) a method for dehydrating organic electrolyte solutions using a dehydrating adsorbent such as a molecular sieve or finely divided alumina (JP-A 10-334730);    (2) a method, similar to that commonly used for polymer dehydration, which involves warming, then dehydration by means of distillation or azeotropic boiling (JP-A 11-217350).
However, dehydration methods which are carried out using dehydrating adsorbents such as molecular sieves or finely divided alumina impart adverse electrochemical effects on account of, for example:    (1) the residual presence of co-solvent used to lower the viscosity;    (2) the introduction of impurities from the molecular sieve or finely divided alumina;    (3) the adhesion and remaining presence of molecular sieve or finely divided alumina on the electrolyte, negative electrode surface or separator; and    (4) lack of a sufficient decrease in the moisture content.Dehydration by means of distillation also presents serious drawbacks.    (1) The temperature and time requirements for such distillation invite reactive double bond-bearing compounds to begin polymerizing.    (2) In the case of azeotropic distillation, traces of the organic solvent used as the entrainer remain behind, lowering the electrochemical properties of the polymer gel electrolyte.